n an interview on Monday, the day after his bank agreed to take over Bear Stearns at a fire-sale price. “I can’t even describe the seriousness of that. I always talk about how bad things can happen that you can’t expect. I didn’t fathom this event.”

TWO months before he resigned as chief executive of Citigroup last year amid nearly $20 billion in write-downs, Charles O. Prince III sat down in Washington with Representative Barney Frank, the chairman of the House Financial Services Committee. Among the topics they discussed were investment vehicles that allowed Citigroup and other banks to keep billions of dollars in potential liabilities off of their balance sheets — and away from the scrutiny of investors and analysts.

“Why aren’t they on your balance sheet?” asked Mr. Frank, Democrat of Massachusetts. The congressman recalled that Mr. Prince said doing so would have put Citigroup at a disadvantage with Wall Street investment banks that were more loosely regulated and were allowed to take far greater risks. (A spokeswoman for Mr. Prince confirmed the conversation.)

It was at that moment, Mr. Frank says, that he first realized just how much freedom Wall Street firms had, and how lightly regulated they were in comparison with commercial banks, which have to answer to an alphabet soup of government agencies like the Federal Reserve and the comptroller of the currency.

“Not only did Wall Street have so much freedom, but it gave commercial banks an incentive to try and evade their regulations,” Mr. Frank says. When it came to Wall Street, he says, “we thought we didn’t need regulation.”

In fact, Washington has long followed the financial industry’s lead in supporting deregulation, even as newly minted but little-understood products like derivatives proliferated.

During the late 1990s, Wall Street fought bitterly against any attempt to regulate the emerging derivatives market, recalls Michael Greenberger, a former senior regulator at the Commodity Futures Trading Commission. Although the Long-Term Capital debacle in 1998 alerted regulators and bankers alike to the dangers of big bets with borrowed money, a rescue effort engineered by the Federal Reserve Bank of New York prevented the damage from spreading.

“After that, all was forgotten,” says Mr. Greenberger, now a professor at the University of Maryland. At the same time, derivatives were being praised as a boon that would make the economy more stable.

Speaking in Boca Raton, Fla., in March 1999, Alan Greenspan, then the Fed chairman, told the Futures Industry Association, a Wall Street trade group, that “these instruments enhance the ability to differentiate risk and allocate it to those investors most able and willing to take it.”

Although Mr. Greenspan acknowledged that the “possibility of increased systemic risk does appear to be an issue that requires fuller understanding,” he argued that new regulations “would be a major mistake.”

Mr. Greenberger, still concerned about regulatory battles he lost a decade ago, says that Mr. Greenspan “felt derivatives would spread the risk in the economy.”

“In reality,” Mr. Greenberger added, “it spread a virus through the economy because these products are so opaque and hard to value.” A representative for Mr. Greenspan said he was preparing to travel and could not comment.

A milestone in the deregulation effort came in the fall of 2000, when a lame-duck session of Congress passed a little-noticed piece of legislation called the Commodity Futures Modernization Act. The bill effectively kept much of the market for derivatives and other exotic instruments off-limits to agencies that regulate more conventional assets like stocks, bonds and futures contracts.

Supported by Phil Gramm, then a Republican senator from Texas and chairman of the Senate Banking Committee, the legislation was a 262-page amendment to a far larger appropriations bill. It was signed into law by President Bill Clinton that December.

Mr. Gramm, now the vice chairman of UBS, the Swiss investment banking giant, was unavailable for comment. (UBS has recently seen its fortunes hammered by ill-considered derivative investments.)